An x band reflectarray antenna using concentric rings and a cross

An X Band Reflectarray Antenna Using Concentric Rings and a Cross An X Band Reflectarray Antenna Using Concentric Rings and a Cross Tran Nguyen Thi Nhat Le1, Hoang Dang Cuong1, Nguyen Thanh Tam2, Minh[.]

2021 8th NAFOSTED Conference on Information and Computer Science (NICS) An X-Band Reflectarray Antenna Using Concentric Rings and a Cross Tran Nguyen Thi Nhat Le1, Hoang Dang Cuong1, Nguyen Thanh Tam2, Minh Thuy Le3, Nguyen Quoc Dinh1* Quy Don Technical University, Ha Noi-City, Viet Nam, E-mail: trannhatle.k47@gmail.com, cuongtdc2@gmail.com, dinhnq@lqdtu.edu.vn 2Technical College of Communications, Ha Noi-City, Viet Nam, E-mail: nguyenthanhtam85287@gmail.com 3School of Electrical Engineering, Hanoi University of Science and Technology, Ha Noi-City, Vietnam, E-mail: thuy.leminh@hust.edu.vn (*Corresponding author: Nguyen Quoc Dinh) 1Le Abstract - In this work, an X-band reflectarray antenna using two concentric rings and a cross is proposed Three types of unit cells are designed and analyzed to improve the phase sensitivity of the unit cell A center-feed reflectarray antenna with dimensions of 205.7 mm x 205.7 mm is then designed It performs an excellent gain of 26 dBi at 10 GHz It also achieves a good aperture efficiency of 67%, a sidelobe level of -15.6 dB, and a 1-dB bandwidth of 12% Keywords - Reflectarray, ring, cross, wideband, phase sensitivity I INTRODUCTION The reflectarray antenna (RFA) was firstly introduced in 1963 [1] However, until the late 1980s, it was paid more attention thanks to the development of the printed antennas Compared to the conventional reflectors, the microstrip reflectarray has many distinctive advantages, such as high efficiency, planar, low cost, low profile, easy fabrication, and capability of reconfiguration [2-4] RFAs also tackle the high loss in the feeding network in the phase arrays by air feeding, especially at millimeter-wave Moreover, they not require numerous RF transmit/receive modules which are expensive and cause the systems more complicated Hence, it could be a candidate for a replacement of the phased arrays antennas in satellite systems, radars, point to point terrestrial links [5-7] However, RFAs have a limited bandwidth, which is inherited from microstrip antennas [8] Moreover, the air feeding structure causes differential spatial phase delay, which degrades their bandwidth, especially for moderate and large sized reflectarrays [8] The bandwidth of a RFA is affected by two components: the bandwidth of the unit cells and the structure of the reflectarray that cause the spatial dispersion As the spatial dispersion is a principle limitation, many researchers have mainly focused to improve the BW of the unit cell which is mainly depended on the phase curve, and changes rapidly around the resonant frequency, leading to the phase sensitivity If the phase sensitivity is too high, the etching tolerance can cause phase errors To lower the phase sensitivity, a thicker substrate can be adopted, but, it leads to a low phase-shift range and high cost to the antenna Recently, various techniques have been adopted to broaden the BW of the unit cells, which are multi-structures in [9-11], or multilayer [12, 13] Besides, the reflectarrays using sub-wavelength elements achieve a larger bandwidth [14-17] In this work, the authors present a reflectarray antenna with 256 elements, operating at X-band Three types of unit cells are analysed and from them, the third unit cell is proposed to improve the BW of the reflectarray This novel multi-structures unit cell consists of two concentric rings inside and a cross outside A foam layer is added to reduce the 978-1-6654-1001-4/21/$31.00 ©2021 IEEE phase slope By this way, the phase slope is significantly decreased, but the phase range is still kept at 3600 The paper is divided into four sections Section II shows the progress of analyzing and designing the three unit cells and the performances of these unit cells Section III presents the configuration of the proposed reflectarray antenna and simulation results The conclusion of this work is given in section IV II UNIT CELL DESIGN To improve the phase sensitivity of the unit cell for the reflectarray, three types of unit cells are designed, simulated and analyzed The topologies of them are illustrated in Figure Figure 1a presents three types of unit cell (front views and side views) with their dimensions, which are analyzed Figures 1b and 1c illustrate the procedures of changing the size of the first unit cell and second and third unit cells respectively As presented in Figure 1a, all three types of the unit cells are etching on the substrate Diclad 880 with permittivity ɛr = 2.2 The first unit cell is just a cross etching on the substrate, while both cross and rings are adopted for the second and the third Compared to the second, the third unit cell is placed on a layer of ROHACELL with a dielectric constant of 1.06 The detailed dimensions of the three types of the unit cells are shown in Table I The unit cells are simulated utilizing a Floquet cell [18] provided by the CST Microwave Studio simulation environment It evaluates the reflection characteristics of unit cells in a virtually infinite array which allows calculating the mutual coupling between elements The operating principle of Floquet port is described in Figure The unit cell is located at the end of a rectangular cuboid, which is in the same as the unit cell The plane wave is impinged to the aperture of the unit cell and the wave then reflects back to the wave port with a set of reflection phase and amplitude Explanation of the cell boundary condition and simulation process can also be found in [19] In reflectarray design, every element owned a private phase, which is calculated from the position of the element and the direction of the radiated beam The required phase-shift range of a unit cell for a reflectarray is more than 360o To obtain a required phase for an element, varying the size of elements is typically adopted beside other methods such as element rotation and delay line [19] In this work, to ease fabricating, the size of the elements l is adjusted from 5.0 mm to 8.5 mm to achieve the phase-shift ranges for elements in the reflectarray Note that to obtain the phase curves, just the size of element l is adjusted while w1, w2, s1, s2, r1 are constants for all of types of the unit cells Therefore, for the second and third unit cells, r2 is changed proportionally to l 307 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) Type w2 lL r1 w2 s2 w1 Diclad880 h1 r2 Type Ll s1 Diclad880 h1 a (a) a r1 w2 s2 w1 r2 0.1 Reflection coefficient [dB] Type Ll s1 Diclad880 h1 RHC HF h2 Type 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 5.0 (a) Type Type Type 5.5 6.0 6.5 7.0 7.5 8.0 8.5 l [mm] a (b) (b) Type and type Fig The reflection coefficients (a) the reflection phases and phase slopes (b) of three unit cells at 10GHz 300 (c) Fig The unit element: (a) Three types of unit cells (front views and side views) with their dimensions, (b) The procedure of adjusting the size of the first unit cell, (c) The procedure of adjusting the sizes of the second and third unit cells Reflection Phase [deg.] w1 0.2 200 TABLE I DETAIL DIMENSIONS OF THE UNIT CELL w2 s1 s2 r1 h1 h2 0.2 0.2 0.2 2.4 1.575 1.5 Unit: mm Unit: mm 100 -100 -200 -300 -400 Reflection coefficient [dB] 5.0 Fig The model of the floquet port The progresses of changing size of the first unit cell, the second unit cell and the third unit cell are demonstrated in more detail in Figures 1b and 1c respectively The characteristics of three unit cells are simulated by the CST and plotted in Figure In Figure 3, the phase-shift range of the first unit cell is just around 325o, not satisfying the phase-shift range requirement To expand the phase-shift range, the authors adopted resonant multi-structure using two rings and a cross as type in Figure 1a This second unit cell obtains a phaseshift range of 375o However, the trade-off is the significantly high phase slope, around 700o/mm while the figure for the first one is approximately 450o The phase slope is calculated by 8.5 GHz 9.0 GHz 9.5 GHz 10.0 GHz 10.5 GHz 11.0 GHz 11.5 GHz 5.5 6.0 (a) 6.5 7.0 l [mm] 7.5 8.0 8.5 0.0 -0.5 -1.0 -1.5 -2.0 5.0 8.5 GHz 9.0 GHz 9.5 GHz 10.0 GHz 10.5 GHz 11.0 GHz 11.5 GHz 5.5 6.0 (b) 6.5 7.0 l [mm] 7.5 8.0 8.5 Fig The reflection phases (a) the reflection coefficients (b) of the third unit cell from 8.5 GHz to 11.5 GHz equation (1):    ( degree / mm)  l where ψ is the reflection phase, l is the size of the corresponding element 308 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) F To decrease the phase slope or phase sensitivity, a foam layer-ROHACELL is bonded between the ground layer and Diclad substrate layer as type in Figure 1a As a result, the phase slope of this third unit cell drops to 200o/mm and the phase-shift range is still more than 360o Z H D/2 θb θ0 Ri The reflection coefficients of three unit-cells in Figure 3a are higher than -0.7 dB It is acceptable for reflective unit cells Among them, the first unit cell has a higher reflection magnitude but it can not be used for the reflectarray because the phase-shift range is less than 360o Although the second and the third unit cells have similar average reflection coefficients, the third is selected for designing the reflectarray due to the significantly lower phase slope X F’ φb ri Y (x0, y0) D/2 (xi, yi) Fig The geometrical parameters of a planar reflectarray antenna The detailed performance of the third unit cell in the band from 8.5 GHz to 11.5 GHz is presented in Figure The reflection coefficients of elements with sizes of more than 7.5 mm are significantly decreased at the upper band (10.5 GHz, 11 GHz, and 11.5 GHz) At the lower band (9.5 GHz, GHz, 8.5 GHz), although the reflection coefficients show excellent results, the phase-shift ranges are not enough 360o From these results, it can be estimated that the reflectarray using this unit cell has a gain drop at frequencies further from the center frequency REFLECTARRAY CONFIGURATION DESIGN In this work, a reflectarray is constructed from 256 elements with the dimension of 205.7 mm x 205.7 mm The unit elements are arranged with a distance of 12.86 mm (equivalent to 0.42 wavelength at 10 GHz) The feeder is a pyramidal horn, which has dimensions of 42 mm x 38 mm and a peak gain of 13 dBi at 10 GHz Figure illustrates the geometrical parameters of a reflectarray antenna The phaseshifts of elements are calculated by equation (2) [19]:   ( xi , yi )   k0 Ri  R ( xi , yi )    R ( xi , yi )   k0 sin b cos b xi  k0 sin  b sin b yi Fig The front view (a) and the side view (b) of the center-fed reflectarray 10 GHz (horn) -40 30 60 90 120 150 180 Theta [deg.] Fig The radiation pattern of the reflectarray antennas  26.0 25.5 Thanks to the phase curve in Figure 3b, the reflection phases of the elements, which are calculated from equation (3), are convert to the corresponding sizes of the elements on the x-axis Note that the phase curve in this case is of the third unit cell Figures 6a, and 6b demonstrate the elements with various sizes in the reflectarray The distance from the feeder H is mainly depended on the reflectarray antenna aperture, which is similar to the focal length of the conventional reflectors Thanks to [20] and [21], H is calculated for an optimized aperture efficiency and blocking effect In this work, the reflectarray antenna aperture is D = 205.7 mm, the optimized H/D ratio is 0.75, and therefore H =154.3 mm 309 Gain [dBi]  R ( xi , yi )     ( xi , yi )   k Ri 9.5 GHz (x-pol.) 10 GHz (x-pol.) 10.5 GHz (x-pol.) -20 where k0 is the free space wavenumber at the center frequency; Ri is the spatial distance from the feeder (F) to the ith element, R(xi, yi) is the phase of the ith element that creates a radiated beam at (b, b) In this work, just a beam at (b = 0, b = 0) is radiated, therefore:  9.5 GHz (co-pol.) 10 GHz (co-pol.) 10.5 GHz (co-pol.) 20 Gain [dBi] III 25.0 24.5 24.0 23.5 23.0 9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8 Frequency [GHz] Fig Gain against frequency of the reflectarray antennas 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) The performance of the reflectarray is shown in Figures 7, and Table II As shown in Figure 7, the reflectarray achieves a maximum gain of 26 dBi at 10 GHz while the sidelobe level is around -16.5 dB The cross-polarization level at 10 GHz is -36.9 dB Figure shows the obtained 1dB bandwidth is 12%, from 9.5 to 10.7 GHz As shown in Table II, the gain drops more and more at frequencies further the center frequency, which reflects the performance of the third unit cell A comparison with other reflectarray antennas using concentric rings is listed in Table III The proposed antenna has some good features The sidelobe level of the reflectarray is -1.5 dB lower than others in Table III Moreover, it shows a good peak gain at 10 GHz, which lead to an excellent aperture efficiency of 67% compared to 39.1% in [21], 44% in [22], 33.6% in [23], and 32.2% in [14] However, the bandwidth of the proposed reflectarray is only 12%, just better than the reference [22] The aperture efficiency of the antennas is calculated by equation (4): a  G 4 A (4) where G is the gain of the reflectarray,  is the wavelength of the center frequency, A is the physical area of the reflectarray antenna aperture TABLE II IV CONCLUSIONS An X-band reflectarray antenna with two rings and a cross is proposed in this work The proposed unit cell has multistructures with a foam layer It achieves a low phase sensitivity with a phase slope of 200o/mm and a phase-shift range of more than 360° The reflectarray with 256 elements obtains a peak gain of 26 dBi at 10 GHz It also has a good aperture efficiency of 67% and a cross polarization of -36.9 dB The simulated results show its 1-dB bandwidth of 12% ACKNOWLEDGMENT This research is funded by Hanoi University of Science and Technology under project number T2021-SAHEP-007 REFERENCES [1] D Berry, R Malech, and W Kennedy, 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